Automobile air conditioning systems are used to cool the interior of autos for passenger comfort when outdoor temperatures are high. Automobile air conditioning systems operate by the exchange of heat between a refrigerant, air outside the automobile, and air inside the automobile. The passenger compartment air is cooled by transfer of heat between the compartment air and liquid refrigerant, cooling the air and heating and evaporating the refrigerant. The cooled air is blown into the automobile's passenger compartment by a fan. Heated refrigerant is cooled by transfer of heat between the gaseous refrigerant and air outside the automobile. The gaseous refrigerant is pressurized, then cooled by passing outside air over the refrigerant, thereby condensing the refrigerant and heating the outside air.
Evaporation and condensation of the refrigerant occur in evaporator and condenser coils, respectively. Refrigerant passes through the coils while air passes over and around the coils promoting effective heat exchange. Gaseous heated refrigerant created by heat exchange in the evaporator coil is transferred to the condenser coil for condensation and cooling. The cooled liquid refrigerant in turn, is cycled back into the evaporator coil where the refrigerant is evaporated and heated by cooling the automobile's passenger compartment air.
In typical front engine automobiles the condenser coil is located under the front hood of the car and is exposed to a continuous flow of air from a fan which cools the refrigerant in the condenser coil. However, in some rear engine automobiles, the Volkswagen Beetle in particular, the condenser coil is located beneath the engine, and has no continuous air flow source. When these rear engine automobiles are in motion the condenser receives a flow of air to cool and condense the air conditioner refrigerant. However, when the automobile is stopped, no air flow reaches the condenser, and the refrigerant is not cooled. If the air conditioner is on and the automobile is stopped for a period of time, or if the automobile is subject to frequent stops, the engine overheats and stalls because the air conditioning system overheats from lack of cooling air flow.